This invention is directed generally to methods of measuring corrosion rate in metals. The above-referenced application described measurement of the corrosion rates of cathodically protected systems. In this application, methods are described for extending this analysis to calculation of corrosion rates at any applied potential (cathodic or anodic) or at the free corrosion potential.
In situ electrochemical techniques have already been applied successfully in measuring corrosion rates of freely corroding metals. Analysis of the response of an electrode to small amplitude (&lt;10 mV) sinusoidal voltage perturbations forms the basis of the well established technique known as AC impedance spectroscopy. In studies of corrosion, a small amplitude perturbation is imposed about the free corrosion potential, and the impedance (Z=V/I, where .about. denotes a complex variable, V is voltage, and I is current) is extrapolated to a real value at low frequency to define a "corrosion resistance," R.sub.corr, that is characteristic of a particular electrode/electrolyte combination. The Stern-Geary relationship may be used to calculate the corrosion current, I.sub.corr, as follows: EQU I.sub.corr =.beta..sub.a .beta..sub.c /2.303 R.sub.corr (.beta..sub.a +.beta..sub.c) (1)
where .beta..sub.a and .beta..sub.c are the Tafel coefficients for anodic and cathodic partial reactions, respectively.
The derivation leading to Eq. 1 assumes that the corroding electrode responds linearly to the imposed electrical perturbations; that is, doubling the perturbing voltage amplitude results in a doubled current response (but an unchanged impedance). Since physical variables in all physically realizable systems must have a finite first derivative, it is always possible to achieve linear conditions by applying a perturbation of limitingly small amplitude. The nonlinearity of the current/voltage relationship in corroding systems prohibits the use of conventional AC impedance spectroscopy, either at DC potentials more than a few millivolts from the free corrosion potential or using other than a limitingly small perturbation. The reason is that, in a nonlinear system, the electrical perturbation, which is imposed on the system at a frequency of f, results in a response at .sub.2 f, .sub.3 f, .sub.4 f, etc., in addition to a DC component. Neither the fundamental response (.sub.o f) nor the total power response ##EQU1## can be analyzed to determine the corrosion resistance uniquely. The use of Eq. 1 to determine corrosion rates presumes that the term B=.beta..sub.a .beta..sub.c /(.beta..sub.a +.beta..sub.c) is known. In real systems, B may be difficult to determine without large voltage perturbations which may modify the surface of interest. Even if a value of B is determined by a second experiment, this value may vary during the course of an experiment. Values of R.sub.corr can thus only be used to give a qualitative indication of the corrosion rate.
A number of fairly common situations exist in which the potential of the metal of interest is much more than a few millivolts from the free corrosion potential; in these situations, the Stern-Geary relationship (Eq. 1) is invalid, as discussed above. For example, the well known corrosion control techniques of cathodic protection and anodic protection typically result in a shift in the potential of the metal by several hundred millivolts from the free corrosion potential. Similarly, large shifts in the potential can result when a metal is galvanically coupled to another dissimilar metal. Thus, a technique for monitoring corrosion rates at any potential without the need for separate measurements of .beta..sub.a and .beta..sub.c would be highly useful and have enormous potential for use.